P. Videler

RESPONSE LINEARITY OF NB TUNNEL JUNCTION DETECTORS FOR PHOTON ENERGIES FROM 1.5 TO 6.4 KEV

Recent experimental results show a linear energy response in high quality Nb‐Al‐AlOx‐Nb superconducting tunnel junction detectors for photon energies between 1.5 and 6.4 keV. The experimental data are based on both direct x‐ray illumination and on the escape and re‐absorption of fluorescent photons created in the junction electrodes and in the silicon substrate. The observed linearity of the energy response raises questions on the validity of some theoretical models which describe the relaxation process occurring in a superconducting thin film after x‐ray photoabsorption. Such models generally predict nonlinear effects due to large quasiparticle number densities and short recombination times.Recent experimental results show a linear energy response in high quality Nb‐Al‐AlOx‐Nb superconducting tunnel junction detectors for photon energies between 1.5 and 6.4 keV. The experimental data are based on both direct x‐ray illumination and on the escape and re‐absorption of fluorescent photons created in the junction electrodes and in the silicon substrate. The observed linearity of the energy response raises questions on the validity of some theoretical models which describe the relaxation process occurring in a superconducting thin film after x‐ray photoabsorption. Such models generally predict nonlinear effects due to large quasiparticle number densities and short recombination times.

Quasiparticle diffusion and loss processes in superconducting tunnel junctions

Abstract Superconducting Tunnel Junctions (STJ) are promising as X-ray detectors. However, they still do not reach their theoretical energy resolution due to various loss processes such as the diffusion of quasiparticles out of the electrode volume and trapping in regions of reduced energy gap. Low Temperature Scanning Electron Microscopy (LTSEM) allows the investigation of inhomogeneities in the response of superconducting Nb/AlO x /Nb tunnel junctions with high spatial resolution. In this way diffusion of quasiparticles and local trapping sites can be identified directly. The impact of these processes on the homogeneity of the signal height and the energy resolution can be visualized. Furthermore, the diffusion length and the lifetime of the quasiparticles is derived. Numerical simulations show that it is necessary to further reduce signal inhomogeneities due to diffusion processes and local traps to ensure a mayor improvement in the energy resolution.

Transmission electron microscopy and atomic force microscopy analysis of Nb‐Al‐AlOx‐Nb superconducting tunnel junction detectors

The performance of photon detectors based on superconducting tunnel junctions are related to their current ‐ voltage (I‐V) curve characteristics and, ultimately, to the quality of the thin tunnel barriers (of order 1 nm) which separate the two superconducting thin films. Both the optimization of the spectroscopic performance of these detectors and the development of a reproducible and high yield fabrication route, require a better understanding of barrier quality and growth techniques. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide valuable tools for the investigation of the barrier region and for the control of the quality of the different thin films and related interfaces. In this paper, the results of a TEM and AFM evaluation of Nb‐Al‐AlOx‐Nb tunnel junctions are reported, together with their interpretation on the basis of the I‐V curve performance at low temperature (T≥0.3 K). Thickness disuniformities of the Al plus AlOx overlayer and evidence of barrier defects have...

High-resolution x-ray detection at 1.2 K with niobium superconducting tunnel junctions

X-ray spectra at 6 keV from niobium based superconducting tunnel junctions with highly transmissive tunnel barriers are presented. Signals from the two films can clearly by discriminated by their different temporal and pulse height characteristics. Levels of tunneled charge as high as 2.7 X 106 electrons at 5.9 keV are observed. The best energy resolution obtained at T equals 1.2 K is 53 eV FWHM including electronic noise, for a 50 X 50 micrometers 2 device in a configuration where the x-ray source is collimated to selectively illuminate the center part of the device. Non-linearity is observed which appears dependent on film volume, implying that self recombination may play an important role in these devices. The energy resolution is found to degrade with increasing magnetic field. The spectra from the polycrystalline top film appear significantly degraded if magnetic flux is deliberately trapped in the device.

THE SUPPRESSION OF PHONON INDUCED NOISE IN NIOBIUM SUPERCONDUCTING TUNNEL JUNCTION X-RAY DETECTORS

An investigation into the phonon contamination of x‐ray sensitive superconducting tunnel junctions arising from the x‐ray photoabsorption in various substrates has been conducted. Results are presented on the design of a superconducting tunnel junction (STJ) which substantially reduces or even eliminates phonon induced noise from the substrate. Such noise is the predominant feature in x‐ray spectra from junctions due to the bulk of the photons being absorbed in the substrate rather than in the thin superconducting film. The design involves the choice of a suitable buffer sandwich between the substrate and the STJ. Such a buffer would appear not only to attenuate the phonons created in the x‐ray photoabsorption in the substrate but also to scatter the phonons inelastically, introducing a frequency down‐conversion. Such a process ensures that few phonons of energy sufficient to break Cooper pairs in the superconducting film of the STJ enter the junction.

Investigation of epitaxial niobium based superconducting tunnel junction detectors

Abstract Superconducting Nb/AlAlO x /Nb tunnel junctions (STJs) with epitaxial niobium base electrode are investigated as weak x-ray detectors. The recorded x-ray spectra show a two peak structure that is attributed to the absorption of 6 keV x-rays in the two junction electrodes. Low Temperature Scanning Electron Microscopy (LTSEM) proves, that the high charge output peak of the x-ray spectra is associated with x-rays absorbed in the epitaxial niobium film. The significantly reduced charge output for events in the polycrystalline niobium electrode is found to be bias voltage dependent.

Spatial variations in the X-ray performance of high quality superconducting tunnel junctions

The results on the illumination of Superconducting Tunnel Junctions with highly collimated X-rays are presented. The12µm square junction was of extremely high quality with a very low thermal current at temperatures of about 1K and a highly transmissive barrier. The 6keV X-rays were precisely collimated to a spot of about10µm diameter. The results clearly show that different parts of the junction respond to X-rays differently, introducing a range of charge components in the spectrum. An energy resolution of ∼100eV was achieved for the collimated illumination of the top Nb film.

UV to IR photon detection using superconducting tunnel junctions

Superconducting Tunnel Junctions have been investigated in the past as particle and X-ray detectors. We report here on results obtained illuminating Nb-Al-AlOx-Nb junctions with radiation covering the UV to IR (250 – 1100 nm) at 0.30 K. In the presence of continuous illumination, an increase of the subgap current is observed, while the change is found to be consistent with the amount of energy supplied to the junction. Several measurements have been performed in a pulsed mode, showing the detection of pulses containing about 100 photons each, at λ=850 nm. These results have allowed a determination of the mean energy required to create a single charge carrier in Nb, which agrees well with theoretical predictions. Such results open new possibilities for the fabrication of optical photon counting cameras based on Josephson junctions.

Low-temperature scanning electron microscopy of niobium superconducting tunnel junctions with trapping blocks

In this paper we investigate the QP transport and loss mechanisms in a superconducting tunnel junction using the Low Temperature Scanning Electron Microscopy (LTSEM) technique. This approach allows precise control of the energy and position of a deposited electron pulse, which, within certain conditions, simulates the X-ray photo-absorption process. An Nb-Al-AlOxNb junction designed as a test structure which included Al blocks in the leads to act as quasiparticle traps, has been investigated. Asymmetries in the LTSEM signal distribution can be qualitatively described from the device geometry. From both the spatial distribution can be qualitatively described from the device geometry. From both the spatial distribution and the time resolved measurements the diffusion length in the amorphous Nb was determined to be of the order of 8 micrometers . The LTSEM technique has demonstrated that quasiparticles produced in the leads cannot enter the tunnel barrier due to the presence of the Al traps.

Superconducting tunnel junctions as photon-counting detectors

An experimental analysis of Niobium based superconducting tunnel junctions is presented, evaluating their performance as photon counting detectors. Several mechanism are found to be responsible for the degradation of the energy resolution. In particular, the high magnetic fields necessary to suppress the Josephson current in square junctions are shown to smear the energy bandgap. It is experimentally verified that in junctions with special geometries the Josephson current can be sufficiently suppressed by much lower fields. Several loss and contamination mechanisms are also discussed. Experimental results on new developments, such as quasiparticle trapping blocks, source collimation and substrate buffering, are presented, with a view to demonstrating significant improvements in energy resolution.